The present invention generally relates to power circuit breakers and, more specifically, to a system for checking synchronization of AC voltage sources before closing a power circuit breaker.
FIG. 1 shows a prior art AC electric power system comprising a power circuit breaker 2, a protective relay 4, bus 6, and a pair of voltage transformers 8a, 8b on each side of the circuit breaker. This basic system may be connected to another similar system via a transmission line 10. During faults, including any short circuit, the power circuit breaker 2 opens and interrupts the short circuit current, thereby limiting damage done to the power system. In order to resume power flow across the AC power system, the circuit breaker 2 must be closed. Before the circuit breaker 2 is closed, a synchronism check or sync-check is typically performed by the protective relay 4.
During a sync-check, the protective relay 4 typically compares attributes of the voltages on one side of the circuit breaker 2 to the same attributes of voltages on the other side of the circuit breaker 2 in order to ensure that the voltages of the lines connected to the circuit breaker 2 are synchronized when they are energized. If the circuit breaker 2 is allowed to close when the voltages are not synchronized, equipment damage and power system instability may ensue. Accordingly, the protective relay 4 signals the circuit breaker 2 to close only when the voltages are in synch and the circuit breaker should be closed. If the voltages are not synchronized, signaling from the protective relay 4 to the circuit breaker 2 is prevented.
A sync-check typically compares attributes of the voltages on one side of the circuit breaker to the same attributes of voltages on the other side of the circuit breaker. For example, a sync-check may compare the vector difference of the voltages on the two sides of the circuit breaker. U.S. Pat. No. 6,324,039 describes one method and arrangement in which the difference voltage is representative of the voltage angle of the generator relative to the electrical system. The difference voltage is compared against a reference value, which is selected such that a voltage angle value greater than the reference value will result in damage to the generator and/or electrical system. An output signal is developed which can then be used to trip circuit breakers serving the electrical system if the reference value is exceeded by the difference voltage. Other systems for performing such sync-checks are well known in art.
In systems having three or more voltages connected to each side of the circuit breaker, sync-checks are still performed on a single voltage from each side of the circuit breaker. For example, in a three-phase power system, three voltages would be connected to each side of the power circuit breaker. In this arrangement, usually only one voltage from each side of the circuit breaker is compared.
In order to perform a sync-check, attributes such as magnitudes, relative phases, or slips of at least one voltage from each side of the circuit breaker must be ascertained. For example, referring again to the conventional AC electric power system of FIG. 1, the voltages from both sides of the circuit breaker 2 are measured via step-down voltage transformers 8a, 8b by the protective relay 4 responsible for making the sync-check.
In an effort to save space and costs, it is a common practice that the step-down voltage transformers 8a, 8b only be installed on one side of the circuit breaker 2. For example, FIG. 2 illustrates such an arrangement whereupon a power system comprises two circuit breakers 12a, 12b connected by a power transmission line 14. As in the arrangement in FIG. 1, each circuit breaker 12a, 12b in the prior art arrangement of FIG. 2 is respectively further connected to a bus 16a, 16b and a protective relay 18a, 18b. This circuit further comprises voltage transformers 20a, 20b between the bus 16a, 16b and the protective relay 18a, 18b. Nevertheless, voltage transformers are not installed on the line 14 connecting the two circuit breakers, thereby compromising the ability for the protective relay 18a, 18b to perform the necessary sync-checks. Although economical, this arrangement increases the risk of equipment damage and power system instability due to the inability to perform a sync-check.
Therefore, it is an aspect and object of this invention to provide a system for performing a sync-check despite not having voltage transformers being connected to one side of the circuit breaker.
These and other desired benefits of the preferred embodiments, including combinations of features thereof, of the invention will become apparent from the following description. It will be understood, however, that a process or arrangement could still appropriate the claimed invention without accomplishing each and every one of these desired benefits, including those gleaned from the following description. The appended claims, not these desired benefits, define the subject matter of the invention. Any and all benefits are derived from the preferred embodiments of the invention, not necessarily the invention in general.